Presently, HgCdTe based devices constitute state-of-the-art technology for 8-12 .mu.m detector applications. However, HgCdTe has metallurgical, mechanical and processing problems which limit its application. For example, the material is mechanically brittle, the dopant concentration in HgCdTe is hard to control, it may have long term stability problems, and its electrical properties may be unsatisfactory for 8-12 .mu.m photovoltaic array applications. In addition, the radiation hardness of these devices is quite low.
III-V semiconductor based detectors should be superior to II-VI detectors such as HgCdTe in all of the above mentioned areas. U.S. Pat. No. 4,607,272 of G. Osbourn discloses that the spectral response of HgCdTe devices may be attained with III-V, InAsSb strained-layer superlattices (SLSs). The patent discloses an InAs.sub.1-x Sb.sub.x /InAs.sub.1-y Sb.sub.y SLS electro-optical device with 0.5&lt;x&lt;0.7 and x&lt;y. No mention was made of a high gain photoconductive device because such knowledge of the properties of these materials was not available at the time of the earlier patent. The patent also did not recognize the presence of a type II superlattice.
Other work indicative of prior art includes the observation by G. Dohler, "Doping Superlattices (,n-i-p-i Crystals,)", IEEE Journal of Quantum Electronics, Vol. QE-22, No. 9, September 1986, pp. 1682-1694, of large photoconductive gains in a type II, doping superlattice, short wavelength, GaAs "selective contact" detector. In addition, S. Kurtz et al., "Extended infrared response of InAsSb strained-layer superlattices", Appl. Phys. Lett. 52(10), Mar. 7, 1988, pp. 831-833, discloses that a type-II superlattice occurs in InAs.sub.1-x Sb.sub.x for x&gt;0.6.